FIGS. 7(a) and 7(b) are cross-sectional views illustrating prior art thin film SAW filters. In these figures, reference numeral 1 designates a substrate comprising a material providing a high propagation velocity for an oscillatory wave, such as amorphous Al.sub.2 O.sub.3 (hereinafter referred to as .alpha.-Al.sub.2 O.sub.3) sometimes reflerred to as sapphire. In the structure shown in FIG. 7(a), a piezoelectric layer 2 comprising ZnO or AlN is disposed on the substrate 1. This piezoelectric layer 2 is epitaxially grown on the substrate 1 by vacuum evaporation or sputtering. Multifinger electrodes 3W and 3E comprising a conductive material, such as Al or Au, are. disposed on the piezoelectric layer 2. In the structure shown in FIG. 7(b), the multifinger electrodes 3W and 3E are disposed on the surface of the substrate 1, and a piezoelectric layer 2a comprising an orientation film covers the surface of the substrate 1 including the multifinger electrodes 3W and 3E.
FIG. 7(c) is a top plan view of the SAW filter shown in FIG. 7(a). The multifinger electrode 3W comprises a pair of doublefinger electrodes 3w and 3w' and the multifinger electrode 3E comprises a pair of doublefinger electrodes 3e and 3e'. Reference character d1 denotes the width of the finger portion of the multifinger electrode, and reference character d2 denotes the interval between adjacent finger portions.
A method of fabricating the thin film SAW filter shown in FIG. 7(a) is illustrated in FIGS. 8(a)-8(f).
Initially, as illustrated in FIG. 8(a), the piezoelectric layer 2 is epitaxially grown on the substrate 1 and, thereafter, as illustrated in FIG. 8(b), an electrode metal 3, such as Al or Au, is deposited on the piezoelectric layer 2 by vacuum evaporation.
Then, a resist 4 is deposited over the electrode metal layer 3 (FIG. 8(c)) and patterned (FIG. 8(d)). Using the patterned resist 4a as a mask, the electrode metal layer 3 is etched (FIG. 8(e)), followed by removal of the patterned resist 4a, thereby producing the multifinger electrodes 3W and 3E (FIG. 8(f)). In the step of FIG. 8(e), if the piezoelectric layer 2 comprises an amphoteric oxide, the metal layer 3 must be etched by dry etching, such as ion milling using Ar gas.
A description is given of the operation of the SAW filter.
When a high-frequency signal is applied across the doublefinger electrodes 3w and 3w', an oscillation having a resonance frequency f that is represented by the following equation occurs. EQU f=V.sub.p /2(d1+d2)
where V.sub.p is the propagation velocity of the oscillatory wave, d1 is the width of each finger part of the multifinger electrode, and d2 is the interval between adjacent finger parts.
Only a surface acoustic wave (SAW) having the resonance frequency f is excited due to the resonance effect utilizing the piezoelectric behavior of the piezoelectric layer 2. This oscillatory wave is propagated through the substrate 1 that provides a high propagation velocity for the oscillator wave and reaches the multifinger electrode 3E. The oscillatory wave is converted into an electrical signal by the electrodes 3e and 3e' of the multifinger electrode 3E, whereby only the electrical signal having the excited frequency f is output.
In order to increase the resonance frequency f, it is necessary to increase the propagation velocity V.sub.p of the SAW in the medium. Therefore, the medium should be a material having a sufficient enough piezoelectric property to convert the electrical signal into the SAW and providing a high propagation velocity for the oscillatory wave. However, materials having such properties are limited. Therefore, in the prior art SAW filter shown in FIG. 7(a), the monocrystalline layer 2 comprising ZnO or AlN having a sufficient piezoelectric property is grown on the .alpha.-Al.sub.2 O.sub.3 substrate 1 having no piezoelectric property but providing a high propagation velocity (V.sub.p) for the oscillatory wave, and the multifinger electrodes 3W and 3E are produced on the piezoelectric monocrystalline layer 2. Alternatively, as shown in FIG. 7(b), the multifinger electrodes 3W and 3E are produced on the .alpha.-Al.sub.2 O.sub.3 substrate 1, and the electrodes 3W and 3E are buried in the piezoelectric layer 2a comprising an orientation film.
In the prior art thin film SAW filter shown in FIG. 7(b), since the multifinger electrodes 3W and 3E are located at the interface between the piezoelectric layer 2a and the substrate 1, the SAW can be propagated from the electrodes directly to the substrate 1 which is the propagation medium. Therefore, the conversion efficiency from the electrical signal to the SAW is high, and the transmission loss in the filter is reduced. However, the material of the piezoelectric layer 2a is limited. That is, since a piezoelectric material has a better piezoelectric property as the crystallinity thereof is improved, a monocrystalline layer epitaxially grown on the substrate 1 is desired for the piezoelectric layer 2a. However, when the piezoelectric layer 2a is epitaxially grown on the substrate 1, the crystallinity of this layer 2a is significantly degraded due to the electrodes 3W and 3E on the substrate 1, and the piezoelectric effect is significantly degraded. Therefore, in the structure shown in FIG. 7(b), only an orientation film having poor piezoelectric property is used for the piezoelectric layer 2a.
On the other hand, in the structure shown in FIG. 7(a) in which the electrodes 3W and 3E are disposed on the epitaxially grown piezoelectric layer 2, the excited SAW oscillates mainly in the horizontal direction in the figure, but deviates from the horizontal direction when it is propagated through the piezoelectric layer 2 to the substrate 1, resulting in a significant attenuation of the SAW. That is, the propagation efficiency of the SAW is reduced by the piezoelectric layer 2.
In the fabricating process of the SAW filter shown in FIGS. 8(a)-8(f), when the multifinger electrodes 3W and 3E are produced, since the etching rate of the piezoelectric layer 2 is higher than the etching rate of the electrode metal 3, it is difficult to control the etching. So, the etching technique is limited to dry etching, such as ion milling with Ar gas.